Patterns of user interaction behavior in online social networks have attracted increasing academic interest, since the quantitative understanding of user behavior is helpful to uncover the origins of many socioeconomic phenomena. In order to gain insights into user interaction behavior characteristics in microblogging, we have crawled Sina Microblog data and obtained user profiles and statuses. Six features of users are studied in our experiments, which are followers and friends, statuses and favorites, retweets and comments. Empirical analysis reveals some common statistical features of user behavior in microblogging. First, all of these properties follow different power-law distributions. Especially, followers and statuses exhibit a double-power-law distribution and friends follow a specific interval power law distribution. Second, there exists a strong positive correlation between statuses and followers, retweets and comments, but the relation between followers and friends is weak. Whats more, there exists a moderate correlation between the number of friends and statuses, retweets and followers, and comments and followers. Third, only a small number of statuses have extreme dissemination and concern. From a quantitative view, the result shows that the strong effect of celebrity cannot be ignored in Sina Microblogs. We believe this paper has added rich empirical materials to our knowledge and understanding of user behavior in microblogging.